1. Field of the Invention
The present invention relates to a method for spray drying perishable liquid products, for example milk products, such as skim milk and whey.
2. Description of the Prior Art
The exhaust gas form, a spray dryer normally entrains part of the dried powdered or particulate product and therefore this exhaust gas has to be cleaned before it is exhausted into the atmosphere. This cleaning may, for example, be made by means of a cyclone. However, cleaning by means of a cyclone is quite often not sufficient in consideration of environmental requirements and--when the dried product is rather valuable--not from an economical point of view either.
It is known to further clean the exhaust gas from a spray dryer by passing it through a gas scrubber in which the liquid product which is later to be dried in the spray dryer is used as washing liquid. Thereby it is obtained not only that the product entrained by the exhaust gas of the spray dryer is recovered, but also a heat economical advantage is obtained because before being exhausted into the atmosphere the said exhaust gas will give up part of its remaining heat energy and thereby preheat or concentrate the liquid product which is later supplied to the spray dryer.
In connection with spray drying of milk it has been proposed to use milk in a cold condition, for example at a temperature which is 35.degree.-40.degree. C. below the wet bulb temperature of the exhaust gas. As described in the following such method will, however, not only create serious hygienic problems due to bacterial growth in the gas scrubber system, but the said known method is also uneconomic. The washing liquid introduced into the scrubber will namely always leave the scrubber at a temperature which is adjacent to the wet bulb temperature of the exhaust gas, and because the washing liquid is passed to the scrubber in a cold condition as mentioned above a relatively great amount of the heat energy in the exhaust gas will be used for heating the washing liquid and not for evaporating the same which means that evaporation in the scrubber is reduced.
In connection with spray drying of milk products it is common practice to concentrate the milk products before spray drying, for example in a multistage evaporator. In such known apparatus the washing liquid for the gas scrubber is derived from a supply conduit for the liquid milk product somewhere upstream of the evaporator.
In a known spray drying plant for spray drying of skim milk it has been proposed to use a heat exchanger to exchange heat between the milk supplied to the gas scrubber as washing liquid and the washing liquid leaving the scrubber. In this known plant the milk is supplied to the scrubber system at a temperature of 8.degree. C. and heated to about 38.degree. C. in the heat exchanger. In the scrubber itself the milk takes up part of the remaining heat energy in the exhaust gas from the spray dryer whereby the milk becomes concentrated and further heated to about 42.degree. C. while the milk leaving the gas scrubber passes through the heat exchanger and is thereby cooled from the said temperature to about 10.degree. C. This known plant is described in the Danish periodical Nordeuropaeisk Mejeritidsskrift No. 8 (1975). In the description of this known plant it is pointed out that the temperature at which the milk used as washing liquid is introduced into the gas scrubber is the ideal temperature for bacterial growth. It is maintained, however, that the residence time in the scrubber may be made so short that the inevitable increase in the number of bacteria will not be of substantial importance.
However, experience with the above known method seem to show that it is rather doubtful whether bacterial growth can be suppressed.
Before supporting this statement some bacteriological aspects in connection with conventional production of milk powder by spray drying will be briefly explained. The production in a milk powder factory is normally based on milk supplied from several dairies. In these dairies the raw milk is usually subjected to a pasteurizing treatment--often in connection with cleaning and fat separation--before the milk is sent to the milk powder factory. In case the milk powder factory receives raw milk or milk of poor bacteriological quality this milk will normally be subjected to a pasteurizing treatment upon arrival to the factory. However, during the subsequent storing and treatment of the milk a risk of contamination and bacterial growth will nevertheless exist, and this bacterial growth may even be explosion-like if favourable conditions for bacterial growth exist.
This is mainly due to two facts. Firstly, the natural bactericidal substances present in raw milk are destroyed by the pasteurizing process. Secondly, a phenomenon called "fouling" occurs in apparatuses and plants for treatment of milk. This phenomenon is due to the fact that extremely thin deposits in which bacterial growth may take place are formed on the apparatus surfaces getting in contact with the milk being treated. Therefore, normally it is tried to minimize the time period in which the temperature of the milk is within the range of temperatures most favourable to bacterial growth (normally 30.degree.-50.degree. C.) and to minimize the apparatus surfaces getting in contact with the milk while being within the critical temperature range. When milk is treated in the known apparatus or plant described above the milk passes the critical temperature range no less than three times, namely when the milk is supplied to the gas scrubber, when it is passed from the scrubber, and in the evaporator to which the milk is supplied from the scrubber. Therefore, it is unavoidable that the known apparatus or plant described presents great possibilities for bacterial growth. Of course, similar conditions exist in connection with spray drying of whey and other perishable liquid products.
In some countries the quality requirements to dried milk products are very strict and the unavoidable increase in the bacteria number in the known plants or apparatuses described above results in generation of lactic acid implying a determinable and in certain cases unacceptable deterioration of the finished product.